1. Field of the Invention
The present invention relates to an image coding apparatus for compressing multi-level image data.
2. Description of Prior Art
A block truncation coding is used in the field of compression of the multi-level image data. In the block truncation coding, an image is divided into a plurality of pixel blocks. For each block, a quantity on an average value of the multi-level image data and a quantity on gradation level distribution thereof are calculated, and a pixel data is quantized in accordance with the quantities to determine code data of a smaller number of gradation levels. Then, the quantities on the average value and the gradation level distribution and the code data of each pixel are used as encoded data of the block.
However, when the pixel data is divided into a plurality of pixel blocks and compressed block by block, a great difference between the decoded image data arises at boundaries between the blocks, or a so-called block noise is produced to deteriorate image quality. It is therefore desirable that the block noise is reduced.
For the block truncation coding on image data comprising a plurality of colors for each color plane, it is proposed to shift coding blocks of the plurality of colors. However, this makes the circuit structure complicated because the adjustment is carried out based on correlation in each color.